This invention relates to the sound amplification arts and to their application in the amelioration of auditory deficiencies, both conductive and sensorineural in nature as related to the human ear. It relates particularly to apparatus for correcting deficiencies not only in a person's ability to perceive and comprehend spoken language, but also to achieve a corrective response providing near-normal or normal hearing under all conditions of environmental noise background, acoustic reverberations such as in churches and auditoriums, and perceiving directionality of sounds; i.e., restoration of conditions representative of that response by the normal human ear.
Two types of hearing loss are generally recognized: conductive and sensorineural; however, there are combinations thereof. Conductive losses are generally correctable by means of flat amplification techniques (flat audio frequency response). Sensorineural loss on the other hand is extremely more complex requiring many different combinations of frequency responses (U.S. Pat. Nos. 3,784,750 and 3,784,745) to correct such a hearing loss to near-normal or normal hearing. As many different hearing responses exist as there are variations of corrective lenses for the eyes. Of significance is the fact that many ears have responses analogous to extreme astigmatism in the eyes. The pure conductive losses are statistically very small, on the order of five percent, and many are correctable by surgical techniques. However, the proportion of sensorineural losses is very extensive, with a vast number of such losses resulting from environmental noise damage. Correction of these losses requires very sophisticated fitting techniques, including, of major importance, highly adjustable corrective audio frequency responses.
Previous techniques involving apparatus for correcting deficiencies described in the referenced two patents only partially resolved the problem of providing a corrective hearing response since filter bands were fixed in their bandpass responses and/or insufficient bands were included to provide the optimum corrective response (U.S. Pat. No. 3,818,149).
One difficulty with successful implementation of previous techniques associated with the referenced patents was the inflexibility of achieving the optimum corrective frequency spectrum because of deficiencies in the electronic implementation techniques involved. This arises from the fact that a corrective frequency response for hearing is highly variable from individual to individual depending upon the type of loss; i.e., conductive or sensorineural or combination thereof, and the difficulties in achieving such a corrective response because of many variable factors involved, including size of the ear canal, coupling to the ear through the earmold configuration, and level of perceived amplification. Some people require more power and gain than others for a particular threshold response. New information as published in HEARING INSTRUMENTS (Vol. 33, No. 14, 1982) has shown the significant importance of obtaining acoustical balance between the two ears. If a corrective response is necessary for each ear and if one ear is misfit, it can have a significant imbalance effect and preclude clear speech understanding, particularly in the presence of environmental background noise and reverberations. Thus it becomes of paramount importance to precisely fit each ear with the optimum corrective frequency response and thus preclude difficulties in speech understanding in the real world situations present in normal living. Furthermore, evidence has shown that only through balanced hearing with proper corrective responses can one obtain near-normal overall hearing responses, including music appreciation, directionality of sound, etc.